Tarr — Origin of the Chert in the Btivlington Limestone , 4-31 



silica is deposited) it would exceed the amount of silica depos- 

 ited at the same time only 3-il times. What becomes of this 

 silica? The writer believes the evidence indicates that it is in 

 large part deposited, the deposits being now represented by 

 the chert and flint nodules in limestone, the silica in siliceous 

 limestone, and the colloidal silica in clays and shales. Another 

 part is used by those organisms of the sea which require silica 

 for their tests and other hard parts. If all or even a consider- 

 able part of the silica were to be used by organisms, large de- 

 posits of the siliceous remains of these organisms should be 

 more frequent in our rocks. 



(h) Derived from the land through cheinical denudation. — 

 When rocks decay at the earth's surface those elements are 

 first removed which form salts that are readily soluble in the 

 ground-water solutions. In the case of igneous and crystalline 

 rocks these elements are sodium, calcium, magnesium, silicon, 

 and to a minor extent potassium. In the case of sediments 

 the elements first removed are calcium, magnesium and sodium 

 with some potassium and silicon. The rate of removal w^ill 

 depend upon the rate of disintegration and decomposition of 

 the rocks and these in turn depend upon the physiographic 

 position, the climate, the vegetation, the character of the rocks, 

 etc. The conditions for the most rapid removal of the soluble 

 salts are when mechanical erosion lags somewhat behind essen- 

 tially complete rock decay. 



The amount of silica derived from igneous rocks is usually 

 much in excess of that removed from sedimentary rocks. The 

 average silica content of the streams east of the 100th Meridian 

 which drain areas underlain by sedimentary rocks and glacial 

 drift is 7*8 per cent, while of those streams draining areas of 

 igneous and crystalline rocks it is 28*5 per cent."^ The first 

 figure given is not a great deal larger than the silica content 

 of the Mississippi River at New Orleans, which is 7*05 per 

 cent of a total salinity of 166 parts per million. Similar evi- 

 dence is shown by analyses of streams in western United 

 States. 



As illustrative of the amount of silica available through 

 weathering the following table is useful, as it shows how much 

 silica is freed by the decay of the feldspars : 



Original 

 percentage Percentage Percentage 



of silica lost of kaolin 



Orthoclase 64-86 43*24 46'36 



Albite 68-81 45-0'7 49-10 



Leith and Mead estimate that there is 23'17 per cent of 

 silica freed by the alteration of the average igneous rock. 

 Under conditions of complete chemical decomposition practi- 



* R. B. Dole, W. S. Paper 236. 



